Gecko vision--retinal organization, foveae and implications for binocular vision.

Geckos comprise both nocturnal and diurnal genera, and between these categories there are several transitions. As their retinae have definitely to be classified as pure cone retinae, they provide an especially attractive model for comparison of organization and regional specializations adapted to very different photic environments. While the visual cells themselves show clear adaptations to nocturnal or diurnal lifestyles, the overall retinal organization is more related to that of diurnal vertebrates. Nocturnal geckos have lost any foveae of their diurnal ancestors, but they have retained a low convergence ratio and a high visual cell density. To enhance visual sensitivity, they exploit binocular - but not necessarily stereoscopic - vision. Diurnal species have retained binocular vision. Most diurnal species have developed new foveae, which are consequently located not in the central but in the temporal region of the retina.